1. Field of the Invention
The present invention relates generally to a battery charger for nickel-metal-hydride batteries or other secondary batteries, and more particularly, to a battery life judging device employed in the battery charger.
2. Description of the Related Art
Rechargeable batteries are commonly used as a power supply for portable devices. When a battery in such a device loses its capacitance, the battery is removed from the portable device, charged with a charging device, and subsequently remounted in the portable device. By repeating this process, the rechargeable battery can be used numerous times. However, as the charging/discharging cycle is repeated, the discharge capacity of the battery declines, as shown in FIG. 5. Since the initial properties of the battery deteriorate in this way, there is a limit to the number of times that the battery can be charged and discharged. Conventionally, users have had to determine when the battery life has expired on a case-by-case basis using their own experience or the like. Consequently, there has been demand for a technique to learn when the life of a battery has expired. To meet this demand, Japanese Patent No. 3,336,790 proposes a battery life judging device for determining when the life of a battery has expired based on whether the battery voltage exceeds a prescribed life judging threshold determined corresponding to the number of battery cells making up the battery.
FIG. 6 shows the battery voltage and charge current when charging a battery that operates normally. As shown in FIG. 6, the battery voltage rises gently in the initial charging phase when the battery is charged with a constant level charge current. FIG. 7 shows the battery voltage and charge current when charging a battery that has deteriorated. As shown in FIG. 7, the battery voltage rises abruptly in the initial charging phase when the battery is charged under the same condition as in FIG. 6. Hence, the battery life judging device according to Japanese Patent No. 3336790 determines that the life of this battery has expired because the abrupt voltage rise in the initial phase exceeds the prescribed life judging threshold.
However, since batteries exhibit different voltage behavior according to the type of battery, internal material, manufacturing method, and the like, it is often difficult to judge whether the battery life has expired based simply on whether the battery voltage exceeds the life judging threshold. For example, in some types of batteries, the voltage does not rise abruptly in the initial phase of charging regardless of whether the battery is normal (healthy) or deteriorated. For batteries that exhibit this property, the life of the battery cannot be determined simply by judging whether the battery voltage has exceeded the life judging threshold.